Well, Tua was essentially a Marciano clone, heavier but with inferior skill, conditioning, workrate, and two-handed power. Tua very arguably beat Ibeabuchi(all of the commentators and many fans had Tua winning that fight). I imagine that Marciano takes a close, hard-fought decision in a 12-rounder, possbly a late stoppage in a 15-rounder depending on how good Ibeabuchi's 15-round stamina was.